关于动词的省略，求指点

angelcity

OG 87:In an effort to reduce their inventories, Italian vintners have cut prices; their wines are priced to sell, and they do (sell)
OG89: Dirt roads may evoke the bucolic simplicity of another century, but financially strained townships point out that dirt roads cost twice as much to maintain as paved roads (cost)

我迷惑的是,87题中为什么do后面省略的是sell而不是其他动词，比如priced,而89题中省略的又是cost，而不是其他动词（maintain）？请指点啊，谢谢~~~~

fdrandom

87从意思上理解，降价为了sell，然后do sell了。
89比较结构要对等，as paved roads cost to maintain